In the manufacturing of circuit boards, the following methods have been generally used to fill electrically conductive paste into penetrations for through holes and openings for via holes (VH) in the circuit boards. First, conductive filler and resin particles are added to conductive metal particles, the resultant of which is mixed into solvent to make a mixture, and then the mixture is agitated into a conductive paste. Thereafter, a mask is placed on the penetration or opening positions, and then, from the top of the mask, the conductive paste is filled into the penetrations or openings with a squeegee by the method of printing. Alternatively, another method directly fills the conductive paste into the penetrations or openings with a dispenser.
In the filling of the paste by such methods, there has been a problem that air bubbles may be trapped into the conductive paste during the agitating or printing described  above. This problem is remarkable in the filling of a highly-viscous conductive paste containing much conductive filler, then resulting in air bubbles remaining in the filled conductive paste. When the paste is cured in the state of still containing air bubbles, fears are in that the interlayer connection resistance may become unstable and/or water may be accumulated in the air bubbles. Particularly, if the paste retains moisture, there is the risk of a water vapor explosion under a high temperature atmosphere in reflow soldering process.
Further, the larger the amount of conductive filler contained in such conductive paste is, the higher the viscosity of the paste is. Therefore, when a highly viscous paste containing relatively large amount of conductive filler is filled into an opening, the resultant depression on the surface is small in depth. However, air is easily involved into the paste during kneading and filling, thereby resulting in a problem that air bubbles are easy to remain in the filled conductive paste. On the other hand, when a low viscous paste containing relatively small amount of conductive filler is filled into an opening, the involving of air into the paste during kneading and filling is reduced, but there is a problem that the resultant depression on the surface is large in depth. If air bubbles remain in the filled paste, they may contain moisture. In this case, after incorporated into electronic parts, the air bubble-containing paste may cause delamination in heat treatment such as reflow soldering, thereby resulting in degradation in connection resistance. Further, if the  depression on the surface is large in depth, the filled conductive paste can not be sufficiently compressed during lamination press. That is, although the conductive paste is depressed downward by the thickness of the bump, the amount of depression is so small that the conductive filler can not closely contact with each other, thus easily causing a problem of degradation in connection resistance.
When a conductive paste in the center of VH's is also scraped off by a squeeze in the stage of squeezing treatment in printing and filling the conductive paste into the interior of the VH's, the problem concerning a depression in the surface can occur. Thereupon, when circuit boards are laminated to each other, the contact area between the conductive bump of this circuit board and a conductor circuit of a circuit board laminated on the top side of this circuit board can become small. Also, the bonding agent can be bit into between the conductive bump and the conductor circuit. Further, when the conductive paste contains volatile solvent components in large amount, the volume of the conductive paste reduces in the stage of pre-curing the conductive bump, thus causing a similar problem.
Also, conventional conductive pastes use liquid epoxy resins, as disclosed, for example, in Japanese Patent Laid-Open No. 10-60319. This is because the excellent flow property of material quality is preferred for filling a conductive paste into a small opening and, at the same time, the use of solvent is avoided to prevent air bubbles from entering the opening. After this conductive paste is filled into the opening in the  circuit board, it is pre-cured to B stage, then proceeding to a next treatment process.
On the other hand, in the manufacturing of multilayer circuit board, for the purpose of making a connection between respective circuit boards, the conductive bumps are sometimes protruded from the circuit board. When the conductive bumps are made of a conductive paste, the conductive paste requires the following properties; (1) being in the state of moderate hardness when the conductive bumps have been formed by the conductive paste, (2) softening temperature thereof higher than that of an bonding-agent for bonding the respective circuit boards during hot-press, and (3) glass transition temperature thereof higher than 150° C., and the like.
However, because conventional conductive pastes do not satisfy the above properties, they can not be used for manufacturing the conductive bumps.
In response to the requirements of miniaturization, lighter weight, higher speed, and higher performance for recent electronic equipment, there is proposed a multilayer circuit board having an interstitial VH structure (hereinafter, sometimes abbreviated to IVH structure), which is easily adaptable to the higher density of wiring, instead of multilayer circuit boards having conventional through-hole structures. In the multilayer circuit board having this IVH structure, each interlayer insulating layer making up a laminated body is provided with a VH for making an electrical connection between conductor circuits. Such a circuit board is characterized in that an electrical connection between  conductor circuit patterns in one layer and an electrical connection between a conductor circuit pattern in one layer and a conductor circuit pattern in another layer are made through a VH which does not pass through the wiring board (buried VH or blind VH). Therefore, the multilayer circuit board having such a IVH structure does not require a special area for forming a through hole, and thus each interlayer connection can be made by only a fine VH. Thus, it promises the easy realization of the miniaturization and higher density of electronic equipment and the high-speed propagation of signals.
However, in the manufacturing process of the multilayer circuit boards having the above-described IVH structure, there have been problems caused by an uncured resin used as an insulating resin substrate, such as glass epoxy preapreg obtained by impregnating glass fabric with a epoxy resin. That is, a plurality of circuit boards are formed by bonding copper foil to the preapreg with hot-press and etching the copper foil to form a conductor circuit, and then the plurality of circuit boards are laminated via a bonding agent. Thereafter, when the laminated circuit boards are hot-pressed at a time into a multilayered board, it has been observed that the shrinkage of the cured resin causes displacements of the positions of VH's in the X and Y directions. In order to deal with such displacements, the diameter of via lands has been required to be made large in advance, thereby making precision wiring difficult. Concerning this problem, the inventors proposed a single-sided circuit board or double-sided circuit  board manufactured by using a resin substrate made up of a cured resin as a core material, instead of conventional insulating substrates made of uncured resins, and forming a conductor circuit or conductor circuits on a single side or both sides of the core material, and then making connections between the conductor circuits through filled VH's. Thus, the inventors provided a method for manufacturing a multilayer circuit board by conveniently combining and laminating a plurality of these boards and then hot-pressing the laminated boards at a time (Japanese Patent Application No. 10-179192).
However, this proposal has the following problem to be solved. That is, in the manufacturing of the double-sided circuit board, a through hole is formed in an insulating substrate, and a filled VH is formed by filling a conductive paste into the through hole by printing or the other methods. After that, copper foil is bonded to both sides of the substrate and the copper foil is etched, thereby forming conductor circuits on both sides.
However, when a multilayer circuit board is manufactured by laminating such double-sided circuit boards and single-sided circuit boards, the double-sided circuit boards and single-sided circuit boards must be manufactured in separate manufacturing lines. Therefore, there remains a realistic problem of the increased cost of manufacturing of the multilayer circuit board.